1. Field of the Invention
The present invention relates to an actuating system for propellers with variable or controllable pitch.
2. The Prior Art
Marine propellers with controllable pitch have long been known to the art and are used to a significant extent both as forward propulsion propellers and steering propellers, and in so-called thrusters. In the majority of cases the pitch of such propellers is controlled and varied by means of an hydraulic actuating system comprising an hydraulic master servomotor which is connected mechanically to the propeller blades and is effective for altering and controlling the pitch of the blades. The master servomotor can be placed at various locations in the system, although in the majority of cases it is placed in the propeller hub, the servomotor usually comprising a hydraulic piston-cylinder device, the piston of which is connected to the propeller blades for adjusting the pitch angles thereof. Pressure medium is delivered to the master servomotor from a source of pressure medium incorporated in the actuating system, under the control of a main control valve. The main control valve is also often installed in the propeller hub. The valve is actuated by means of a valve rod which extends through an axial bore in the propeller drive shaft and is connected to an auxiliary servomotor, located inside the vessel to which the marine propeller is fitted, such that the valve rod, and therewith the main control valve, can be actuated by means of the auxiliary servomotor. The supply of pressure medium to the master servomotor and to the main control valve in the propeller hub is effected through a so-called pressure-oil box or oil distribution box connected to the pressure medium source and embracing the propeller drive shaft, or a shaft connected thereto, inside the vessel. The oil distribution box incorporates a stationary pressure-medium chamber which is connected to the source of pressure medium and which surrounds the rotatable propeller shaft and is sealed against the outer cylindrical surface thereof. The pressure-medium chamber connects, via a radial channel in the propeller shaft, with a channel which extends axially within the shaft and out into the propeller hub, where said channel communicates with the master servomotor via the master servomotor main control valve, which is also located in the hub. The axial channel in the propeller shaft is often placed within the valve rod of the main control valve, in which case the valve rod has the form of a hollow tube. The auxiliary servomotor located inside the vessel is actuated by means of an auxiliary control valve, which is normally operated electrically, but which may also be pneumatically, hydraulically or even mechanically operated. The auxiliary servomotor control valve is normally actuated by means of a lever positioned on the bridge of the vessel, whereby the pitch of the propeller blades can be altered and set by an appropriate activation of the lever. The auxiliary servomotor and its control valve are normally positioned adjacent to or on the pressure-oil box. The main control valve for controlling the master servomotor located in the propeller hub need not necessarily be located within the propeller hub, however, but may be placed at some location inside the vessel, for example connected between the pressure-medium source and the pressure-oil box. In this case no auxiliary servomotor is needed for activating the main control valve, which consequently obviates the need for an auxiliary control valve for such an auxiliary servomotor, it being possible in this case to control the main control valve directly through the lever located on the bridge of the vessel. The pressure-oil box may also be placed within or on the outside of a gear mechanism connected to the propeller drive shaft, when such a mechanism is provided.
An hydraulic actuating system of the kind described above for altering the pitch of a controllable pitch marine propeller is prone to various faults which are liable to render the system non-functionable, either in part or in total. It must be possible in such cases of system malfunction, however, to influence the blade pitch, at least to some extent, and hence a number of back-up or emergency systems are normally installed. For example, there is normally provided a back-up communication between the bridge or a control room and the auxiliary servomotor control valve or a back-up therefor, which can be used in the event of an interruption in the normal connections between the bridge and the auxiliary servomotor control valve located in the vicinity of the oil distribution box. Such back-up systems often also include press-buttons arranged on or close to the oil distribution box, these buttons being usable for actuating or controlling the system. In order for such back-up systems to function satisfactorily, however, it is necessary for the hydraulic system as a whole and for the mechanical components of both the propeller and the oil distribution box to be free from faults. Consequently, it is standard practice to provide controllable pitch propellers with some form of emergency device for use in the event of a faulty hydraulic system. Such an emergency device may comprise, for example, an arrangement of springs in the propeller hub, these springs being operative in urging the propeller blades to a position corresponding to maximum or full pitch, when the propeller shaft is non-rotating, whereafter the propeller can be driven with this maximum pitch at a reduced speed. In another alternative emergency arrangement, the valve rod of the main control valve serving the master servomotor and housed in the propeller hub is arranged in a manner which enables it to be connected mechanically to the piston rod of the main servomotor, whereby with suitable arrangement the piston rod can be moved, through the intermediary of the valve rod, to a position corresponding to full or maximum pitch, whereafter the propeller can be driven at this pitch at a reduced speed. All known emergency actuating systems of this kind, however, are encumbered with the disadvantage of having limited mechanical strength and of being incapable of setting the blade pitch to more than one single, given position, this position normally being a full-ahead position.
It is often desirable, however, to be able to set the propeller blades to any pitch whatsoever with the aid of an emergency actuating or control system, although this has not been possible with the actuating systems known hitherto. It is also desirable to be able to drive the propeller at any speed whatsoever when setting the pitch by means of an emergency actuating system.